As a new type of solid state light source, the LEDs are hopeful to become a new generation of illumination source due to the advantages such as energy saving, environmental protection and long lifetime. It is well known that almost all of the existing LEDs are driven by the direct current, while the electricity for production and household is alternating current. Thus, the LED product used at present needs a power converter to convert alternating current into direct current. The introduction of the power converter brings many negative effects. Firstly, the service life of the power converter is far less than that of the LED, thus the service life of the illumination device is shortened. Secondly, the power converter decreases the efficiency of the light emitting device. Thirdly, in the small power applications, the power converter decreases the power factor and increases the total harmonic distortion of the current. In order to sufficiently exert the advantages of the semiconductor illumination, the LED light emitting device that can be directly driven by the alternating current becomes a research hotspot at present
Most of the existing disclosed technologies of alternating current LED connect a plurality of LED modules in inverse-parallel, or in the circuit topological structure of a bridge rectifier, so as to meet the driving requirement of the alternating current. But the alternating current fluctuates periodically in a certain frequency, and the LED itself has a turning-on voltage, so the LED is turned on to emit light only when the instantaneous voltage exceeds the turning-on voltage, otherwise the LED is turned off and no light is emitted. Such a circuit leads to a very low luminescence efficiency of the LED, and flickers occur along with the fluctuation of the voltage of the alternating current.
The international patent WO2004/023568A1, entitled as “LIGHT-EMITTING DEVICE HAVING LIGHT-EMITTING ELEMENTS”, proposes to integrate LED chip arrays on a sapphire substrate, so as to provide a light emitting device driven by the alternating current, but the problem of the LED flicker is not solved
U.S. Pat. No. 7,489,086 B2, entitled as “AC LIGHT EMITTING DIODE AND AC LED DRIVE METHODS AND APPARATUS”, provides an alternating current LED device, which encapsulates a plurality of LEDs integrally, so as to compensate for the LED flickers caused by the alternating current with the vision persistence effect of the human eyes. But the patent does not eliminate the flickers caused by the periodic fluctuation of the voltage of the alternating current.
It is clear that all the disclosed LED alternating current driving technologies have a defect that the LED driving current fluctuates with the voltage of the alternating current, thus the brightness varies when the LED emits light, and the flickers occur. Meanwhile, the core of the LED device is a PN junction diode, whose I-V characteristic is an approximate exponential function, and when the voltages at both ends of the LED exceed the turning-on voltage, the current flowing through the PN junction increases exponentially. The alternating current driving method of the prior art does not use the constant current circuit, and when the temperature of the LED junction rises, the turning-on voltage decreases. But the input voltage is not changed, thus the forward current of the LED increases rapidly, and even the PN junction of the LED may be thermally broken down and then permanently damaged in serious conditions.